1. Field of the Invention
This invention generally relates to an exhaust gas cleaning device for diesel engines of motor vehicles. More particularly this invention relates to a device having a filter capable of physically trapping carbon particulates or the like (hereinafter referred to as exhaust particulates) contained in the exhaust gas and means for burning and removing periodically the caught exhaust particulates, thereby regenerating the capability of the filter.
2. Description of the Prior Art
Exhaust emissions of diesel engines contain considerable amounts of combustible substances, such as carbon particulates, as well as other harmful substances. Hitherto, various kinds of trapping devices have been proposed and used for treating such combustible particulates.
Such trapping devices usually include filter elements, i.e., trapping materials for trapping or catching the exhaust particulates, and means for igniting and burning the trapped exhaust particulates to regenerate the filter elements. As means for igniting and burning the trapped exhaust particulates, an electric heater provided in front of the filter elements is usually used. The heater ignites the trapped exhaust particulates at the upstream end of the filter elements adjacent to the heater. The combustion of the exhaust particulates is then propagated to the trapped articles at the portions of the filter elements downstream from the ignited particulates so that self-combustion of the particulates downstream from the ignited particulates takes place.
If the exhaust gas flows through the filter elements at a high velocity when the heater is electrically energized, the heat of the heater is absorbed by the exhaust gas and the particulates cannot be heated to a temperature high enough for ignition. Even if the particulates are ignited, the high-velocity exhaust gas flow absorbs the heat of the ignited particulates, i.e., of the combustion being propagated, resulting in the failure of complete propagation of the combustion to the particulates located at the downstream end of the filter element. This limits the range of driving conditions of the engine and the periods in which the regeneration of the filter elements can occur.